Windows/Mac/Unix (Thunderbird): The Send Later Thunderbird extension lets you schedule the delivery of emails composed in Thunderbird.

If you're keen on sending emails with Thunderbird's Ctrl-Enter shortcut, then incorporating the Send Later command should be a cinch - just hit Ctrl-Shift-Enter. You'll see the Send this email later dialog, which lets you set the exact minute you'd like the email sent (keep in mind Thunderbird will need to be open at that time). If this seems familiar, it's because it performs the same task advertised by previously mentioned Send Tools, but some readers had trouble getting SendTools to work reliably, and it's not supported in Thunderbird 2.0. Send Later, on the other hand, is 2.0-ready. It's also free, works wherever Thunderbird does.